Safety isolation is an important consideration in the design of any electrical product that can expose its users to potentially hazardous voltage levels. Manufacturers of such products need to ensure that their products are designed to adequately safeguard against electrical shock.
In any safety isolation scheme, there is a certain amount of insulation required to create a protective isolation barrier between primary and secondary circuits. A primary circuit is a circuit that is directly connected to line voltage and therefore has the potential to reach hazardous voltage levels. A secondary circuit is not electrically connected to the primary circuit (that is, there is no conductive connection between the secondary circuit and primary circuit) but could experience hazardous voltage levels if insulation fails.
Underwriters Laboratories Inc. (UL) has developed a safety standard called UL 60950-1 (second edition), which specifies a high level of safety isolation to protect against risk of injury due to electric shock (herein, the “Standard”). The UL 60950-1 Standard is based on the International Electrotechnical Commission (IEC) Standard, IEC 60950-1 (second edition). The UL 60950-1 Standard defines five categories of insulation: functional insulation; basic insulation; supplemental insulation; double insulation; and reinforced insulation. Functional insulation does not protect against electrical shock. Basic insulation is a single level of insulation that provides basic protection against electric shock. Supplemental insulation is independent insulation that can be applied in addition to basic insulation to reduce the risk of electrical shock in the event of a failure of the basic insulation. Double insulation comprises both basic insulation and supplemental insulation. Thus, basic insulation provides a single layer of insulating barrier between primary and secondary circuits, whereas double insulation provides two layers of insulating barrier between primary and secondary circuits. Reinforced insulation is a single insulation system that provides electrical shock protection equivalent to double insulation.
If a secondary circuit is not user accessible, a basic level of isolation protection, that is, basic insulation, may be acceptable. Safety needs for products that include user-accessible secondary circuits, on the other hand, demand two levels of protection provided by double insulation or its equivalent in reinforced insulation.
Although the UL 60950-1 Standard is intended for use with information technology equipment, it has been adopted for many other products and is considered the premier Standard for product designs that require galvanic isolation. One product that requires galvanic isolation is the current sensor.
Some current sensors use one or more magnetic field transducers, or sensing elements (for example, a Hall effect or magnetoresistive transducer) in proximity to a current conductor. The magnetic field transducer generates an output signal having a magnitude proportional to the magnetic field induced by a current that flows through the current conductor. Current sensors with integrated current conductors and magnetic field transducers have a primary conductor (i.e., the current conductor, which has leads connected to a high line voltage via an external current conductor) in close proximity to a secondary conductor (i.e., the lower voltage sensing circuits and signal pins). In applications where the device's primary conductor is connected to line voltages (e.g., 120, 240 or 480 VRMS), maintaining galvanic isolation between the primary and secondary conductors is important.
Some prior magnetic field current sensors are designed to provide basic insulation between the current conductor (which, as mentioned above, connects to line voltage and can be considered a primary conductor or circuit) and other conductive parts of the device that are electrically independent of the primary conductor (e.g., sensing circuits, interconnections and signal pins, which may be considered a secondary conductor or circuit) in the way they are constructed.
Various parameters characterize the performance of such current sensors, including sensitivity. Sensitivity is related to the magnitude of a change in output voltage from the magnetic field transducer in response to a sensed current. The sensitivity of a current sensor is related to a variety of factors. One important factor is the physical separation between the magnetic field transducer and the current conductor. Integration of the current conductor into an IC package allows for close and precise positioning of the current conductor relative to the magnetic field transducer.
The Standard does not specify a minimum thickness through insulation for basic insulation. For insulation to be considered supplemental or reinforced under the Standard, however, the insulation must have a minimum thickness of 0.4 mm. Insulation in the form of a thin sheet material, e.g., “thin film” material, may be used for supplemental and reinforced insulation, irrespective of its thickness, provided that at least two layers of such material are used.
The term “clearance” refers to the shortest distance through air between two conductive parts, such as the primary and secondary leads. The term “creepage” refers to the shortest distance between two conductive parts along the surface of any insulation material common to both parts. The spacing distance between components that are required to withstand a given working voltage (i.e., the highest voltage level that insulation under consideration can be subjected to when a device such as a current sensor is operating under normal use) is specified in terms of creepage and clearance. The creepage requirements and clearance requirements are specified in Table 2N and Tables 2K-2M (of the Standard), respectively. The minimum clearance required for high working voltage ratings is less than the creepage requirement and therefore easier to meet. According to Table 2N, to achieve a rating for an RMS working voltage of 500 V for reinforced or double insulation using a material in material group II, a minimum creepage of 7.2 mm must be achieved.